The continued growth of international travel for business and leisure leads to a significant increase in the number of subscribers roaming in mobile networks, such as UMTS (Universal Mobile Telecommunications system), GPRS (General Packet Radio Services) or GSM (Global System for Mobile communications) networks.
Every time a subscriber moves to a location or routing area served by a different VLR (Visitor Location Register) or SGSN (Serving GPRS Support Node), respectively, the subscriber data are downloaded from the HLR (Home Location Register) in the Home-PLMN (Public Land Mobile Network) to the new network entity serving the user, and deleted in the old VLR or SGSN. In this respect, the term “network entity” is used to designate any network element which may serve a mobile user and which may comprise a database for storing subscriber data. The HLR comprises a database in which all permanent subscriber data and all relevant temporary subscriber data for all mobile subscribers permanently registered in the HLR are stored. The VLR comprises a database in which all subscriber data required for call handling and other purposes for mobile subscribers currently located in the area controlled by the VLR are stored.
In order to support the subscriber identity confidentiality service, the VLRs allocate a unique TMSI (Temporary Mobile Subscriber Identity) to visiting mobile subscribers. Therefore, the VLR is capable of correlating the IMSI of a mobile station and the current TMSI for that mobile station. Similarly, in GPRS networks, a packet TMSI (P-TMSI) is used for identifying a mobile station. However, only a limited number of visiting subscribers can be handled by each VLR.
In case two different overlapping networks have been established by two network operators, e.g. the first network covers a certain small area (e.g. a city) and the second network covers a larger area including the small area, a roaming agreement may be made between the two network operators. Thereby, subscribers of the first network are allowed to get service also outside their home network in the second network. In the following, subscribers located outside their home network are called roaming subscribers.
Roaming agreements between overlapping networks may lead to a problem that one or more VLRs and/or SGSNs of the one network may become overloaded due to a high number of roaming subscribers from the other network. In such a case, new location registration requests for location or routing area updates even from own (i.e. non-roaming) subscribers have to be rejected. This results in a decreased quality of service or service reliability provided by network operators to their own subscribers. Hence, a mechanism is needed to guarantee service for own subscribers.